Martin Models 270 and 275

P6M SeaMaster
Variants/Specifications

In its last major aircraft design, Martin returned to an earlier concept of the flying boat as a bomber. By the end of the 1940's the Soviet Union had tested a nuclear bomb, and the Cold War was in full swing. The newly created Air Force was busy buying and deploying long-range bombers to deliver nuclear weapons, a monopoly viewed by the Navy as unacceptable. Noting the inherent limitations of its force of short-range carrier attack and maritime patrol aircraft, the Navy looked at several means of joining the Air Force as in strategic deterrent. A super-carrier (the United States) was designed to handle larger propeller and jet aircraft then under design. The United States ran afoul of military budget limitations and vehement opposition from the Air Force "bomber lobby." The Navy Bureau of Aeronautics then developed the concept of a "Seaplane Striking Force" centered around the development of large jet-powered seaplanes that could offer performance equal to that of land-based jets. Capable of operating from most of the earth's surface, a small number of these seaplanes could perform mining, conventional and nuclear strike, and photo reconnaissance missions that would complement those of the new Strategic Air Command. With only a tender or submarine needed for re-arming and re-fueling, the SSF promised an economical means of force projection.

Requests to industry were let in April 1951. After a short but fierce design competition with Convair, Martin was awarded contracts for two prototype XP6M-1's, six pre-production service-test YP6M-1's, and up to 24 production P6M-2's. Martin named the SSF aircraft the SeaMaster. The Navy was now in the bomber business.

Design specifications for the SeaMaster were demanding. Required to carry 30,000 pounds of payload to a target 1,500 miles away, the plane was also required to be capable of a high-speed dash at .9 Mach at low altitude. Its hull had to be stressed for open-ocean operations. Design Engineer George Trimble, hydrodynamicist J.D. Pierson, and aerodynamicist J.L. Decker led the design team. Refining work already done on the Marlin's hull design, they adopted a new length-to-beam ratio of 15 to 1 as most efficient in both air and water. The XP5M-1 airframe was rebuilt to test the new hull, redesignated Martin Model 270. Hydroflaps like those on the Marlin were fitted for dual use as air brakes.

A compound turbo/ramjet from Curtiss-Wright was initially designated as the SeaMaster powerplant. After several failures in testing, this engine was dropped in favor of modified Allison J71's, mounted in tandem overwing nacelles. The P6M had the same variable-incidence "flying" T-tail and spoiler ailerons as the XB-51, and its payload was carried in a rotating bomb-bay, pneumatically sealed to be watertight. Swept wings with slight anhedral drooped close enough to the water for wingtip tanks to serve as stabilizing floats, without the drag of struts. The overall result was an airplane with proportions so sleek and simple that they could be described as classic.

The first prototype was rolled out in secrecy on December 21, 1954, and after several months of load-verification tests the XP6M-1 finally took to the air on July 14, 1955, flown by Martin chief test pilot George Rodney. Initial tests revealed only one major problem that required a "fix": the design of the nacelles allowed the afterburner exhaust to scorch and sonically fatigue the rear fuselage. After keeping the plane's development secret, the Navy invited the press for the roll-out in November of the second prototype, which was outfitted with a complete set of navigation and bombing equipment.

All went well with the testing program until December 7, 1955 (two days after the death of Glenn L. Martin), when the first XP6M-1 prototype crashed into the Chesapeake Bay during a routine check ride for the first Navy pilot. All four members of the crew were lost. With no onboard data recorders to help, the accident-investigation team was unable to find a specific fault. Months were lost re-configuring the second prototype with test instrumentation and ejection seats for all the crew. It was not until May, 1956, that flight testing resumed with Ship #2.

By autumn, solutions were being sought for a frequent airframe buzz that plagued both prototypes. One "fix" involved locking the elevators together with the variable-incidence "flying tail." A test flight on November 9 verified that improvement in the vibration, however, in recovering from a shallow dive at high speed, pilot Bob Turner lost pitch control of the aircraft, which started a violent outside loop. The crew ejected safely as the airframe broke up. Information from the flight data recorders indicated that the modified tail configuration had been overpowered by dynamic forces at high speed, due to a previously undiscovered mathematical error in calculating loads for the hydraulic control actuators.

Even at this low point in the program the Navy BuAer still saw promise in the concept and optimistically continued funding for the SeaMaster and a number of expensive "options." A beaching cradle was designed that allowed SeaMasters to taxi in and out of the water on their own power. Two old amphibious-warfare dock ships and two conventional seaplane tenders began shipyard conversions as support ships for the SSF. The submarine U.S.S. Guavina, redesignated as an AO(SS) "oiler," was equipped to refuel SeaMasters at secret seadromes. There were also plans to use an old escort carrier equipped with a retractable rear ramp for "beaching" P6M's, which were too heavy to be hoisted aboard by cranes. Finally, an auxiliary naval air station was refurbished to serve as the SeaMasters' home base; it occupied 1,265 acres at NAS Harvey Point, near Elizabeth City, N.C.

Meanwhile service-test YP's were completed with "fixes" for the problems encountered in the prototypes. Engine nacelles were canted out five degrees from the fuselage and the intakes moved back from the wings' leading edges. Hydraulic control systems were upgraded in the tails. A year after the second crash, the first YP6M-1 was rolled out and flight testing resumed in January 1958. Five other YP's joined the program during 1958, and tests were carried out at a feverish pace. Mine-laying and navigation systems were qualified even though standard Navy mines could not yet withstand sea impact when dropped at high speed. Conventional and "special-weapon" (nuclear) practice shapes were successfully dropped from the rotary bomb-bay, and night and day photo reconnaissance pods were tested.

Early in 1959 production P6M-2's began to emerge from the Martin plant, and the full potential of the design was realized. Installation of newly developed Pratt and Whitney J75 engines gave the P6M-2's nearly 12,000 more pounds of static thrust. This allowed the gross weight to be increased to 195,000 pounds from 171,000 pounds in the YP's. Increased weight meant a greater draft for the hull, which in turn necessitated raising the wing anhedral to zero degrees. Other improvements included full-visibility canopies and transistorized Sperry navigation and bombing systems. Production P6M-2's were equipped with midair refueling probes, and "buddy-pack" refueling kits were designed to fit inside SeaMaster bomb-bays, allowing fast conversion into tankers.

Pilots reported that the planes handled well and were capable of flying Mach .89 "on the deck." This was important, as the development of radar-guided surface-to-air missiles had made low-level flying an essential part of strategic penetration missions. The SeaMaster's wings were especially strong for the extra stress of high speeds through thick air; the aluminum skin at the wing roots was an inch thick. By contrast, the Air Force's B-47 could only manage about Mach .58 at low altitude, the newer B-52 only .55.

By the summer of 1959 all-Navy crews had begun flying three P6M-2's completed so far, and it appeared that operations could begin by early 1960. Rising costs, however, had led to two cutbacks, reducing the number of production items to eighteen, then eight. Then the bottom dropped out altogether. Citing "unforeseen technical difficulties," the Navy cancelled the entire program on August 21.

The decision was and still is highly controversial. More than $400 million had been spent on equipping the SSF, but during its long gestation period newer technologies had emerged. The development of the Polaris ballistic missile and submarine had finally given the Navy its strategic deterrent. Further, the atomic powered carrier Enterprise was going into service with long range nuclear capable strike aircraft, namely, the A3D Skywarriors and supersonic A3J Vigilantes.

Stunned, Martin engineers and executives tried to generate interest in an eight-jet transport version of the P6M, whimsically dubbed the SeaMistress, a huge nuclear-powered flying boat, and a supersonic seaplane somewhat resembling the Air Force Canberra. But there were no takers. Martin Chairman George Bunker announced that the company was now in the missile and electronics business. Fifty years of aircraft design and production was at an end.

Of the SeaMaster program little remains. The aircraft languished on the D Building ramp at Middle River for over a year after the cancellation before being scrapped. The "flying tails" and two rear fuselage sections were sent to Navy test facilities, while two sets of wing floats were used by a Martin supervisor to build a catamaran. Two tails, one fuselage section, and wing floats now belong to the Glenn L. Martin Maryland Aviation Museum.